In a conventional stepper motor system, the driver circuitry and the indexer circuitry is in the form of printed circuit boards (PCB) with integrated circuits and other discrete components soldered to copper traces that are photochemically etched on the board. The driver and indexer circuitry in a stepper motor is typically complex in comparison to the driver and indexer circuitry in non-stepper motor systems. In a PCB configuration, a stepper motor driver is typically large and bulky, thereby requiring a separate enclosure that is unattached to the motor and connected to the motor by a lengthy cable. Connected to the driver is an indexer which tells the driver how far, and at what speed, to advance the stepper motor. In addition, a separate power supply is connected to the driver and indexer.
There are inherent disadvantages in a conventional stepper motor system, however.
1. Size--Conventional driver circuits in a stepper motor system are comparatively large. A high power 1/2 step or micro step drive circuit is bulky and can not be reduced to a small enough size to be placed in a restrictive area or attached directly to a motor. The addition of an indexer or external power supply can significantly add to the size of the system.
2. Weight--When populated with components, the weight of the driver and indexer circuits can significantly reduce the benefits of using a conventional stepper motor system in weight sensitive industries such as the aerospace and aviation industries.
3. Heat Dissipation--The substantial heat dissipation requirements of a conventional stepper motor driver circuit prevents the driver from being mounted in restrictive areas or enclosures without ventilation. Conventional driver circuitry is complex and has many individual components populating a driver circuit, each having its own heat dissipation requirements. Because of the need to dissipate the heat of the individual components through the use of bulky heat sinks, a conventional driver circuit must be large by nature. Even with substantial heat sinking, a conventional driver can only be mounted in low temperature environments.
4. EMI/RF Emissions--Because of the large area on a PCB in which traces and components are exposed, a power switching circuit, such as a driver, will emit EMI/RF that can damage or interfere with EMI/RF sensitive devices located near the PCB. Many industries, such as the health care and semiconductor manufacturing industries, cannot use conventional stepper motor systems because of the EMI/RF noise they emit. In addition, as in the case of a driver located near a stepper motor, EMI/RF produced by an electrical motor may damage or interfere with the exposed EMI/RF sensitive devices located on the driver.
5. High-Frequency Performance--Because of variances in trace width, length and exposure, a PCB circuit is limited in its ability to handle high speed transmission of signals. As in the case when a stepper motor microsteps, a driver with limited high speed signal transmission capabilities would restrict a motor's speed, as well as limit the resolution a motor could possibly achieve. (Example: 50,000 steps per revolution verses 125,000 steps per revolution.) An additional problem occurs when the motor is not in very close proximity with the microsteps driver. The greater the distance that the signal must travel to the motor, the lower the possible speed and resolution a motor can achieve.
6. Circuit Performance--The performance of a PCB driver circuit is limited by the tolerances of the discrete resistors, capacitors, inductors, and traces used in the design, as well as the trace width, length and exposure variances on the PCB. The inability to fine tune the resistors, capacitors, inductors, and traces results in a circuit that is less efficient, less predictable, less reliable, less stable and with broad absolute specifications.
7. Fixed Operating Parameters--In conventional stepper motor systems, the distance between the motor, driver and indexer makes it difficult to monitor small, significant changes in the real time operating parameters, such as temperature and current, occurring at the motor. The inability to monitor these parameters has resulted in the inability to safely optimize a stepper motor's operating efficiency as determined by certain changes in its operating environment.
8. Assembly--Assembling a conventional stepper motor system in a product is time consuming, requiring manual placement and wiring. Errors can frequently occur when placement and wiring is done manually. These errors will often damage the system, and in some cases can cause personal injury.
9. Environment--Conventional driver and indexer circuits are not hermetically sealed. Exposure to humid environments will damage the circuits. In flammable environments, a short in the circuit could cause an explosion.
10. Safety--Damage done to a PCB drive circuit as a result of a hostile environment, or severing the external connecting wires, or inadequate heat dissipation can result in life threatening safety hazards, such as electrocution.
11. Mechanical Stability--In a conventional stepper motor system, the individual components on a driver circuit are not permanently potted in place, and say be become loose as a result of vibration.
In a conventional stepper motor system, the driver circuitry contains a switching means and a translating means. The indexer which is connected to the driver sends pulses to the translator. The pulses and the rate at which the translator receives the pulses determines the distance, speed and direction of a move. The translator then turns on and off the transistors in the switching circuit, thereby controlling the flow of current into the windings of the motor. In some sophisticated configurations, the indexer also contains circuitry for communicating with outside controls, thereby allowing a computer to send variable move commands to the indexer.
In certain "closed loop" applications, an encoder is connected to the stepper motor. The encoder provides feedback to the encoder interface circuitry, which translates the information to a means understandable by the indexer. The information can then be transmitted to the indexer enabling the indexer to correct for errors in the stepper motor's move. Typically the encoder is limited to final position sensing and adjustment.
There appears to be a trend to miniaturize. For example, in a prior art stepper motor system manufactured by Compumotor Corporation of Petaluma, Calif., a combination driver-indexer is combined in a single unit. However, the Compumotor system maintains the limitations of a conventional stepper motor system. Compumotor's driver-indexer combination remains large and heavy, and must remain separated from the motor itself. The combined unit still requires relatively lengthy wires between the driver means and motor. Faulty connections and wiring errors can still occur, possibly damaging the components in the circuit or causing electrocution. The unit still requires an external power supply, which must be manually wired to the unit. In addition, the unit and its associated wiring radiates electromagnetic interference that should be reduced, if possible. Finally, the unit and power supply still requires a relatively large amount of space, as well as the adaptability of the space to handle a significant amount of weight and heat dissipation.
Some companies, such as SGS--Semiconductor Corporation of Phoenix, Ariz.--have developed integrated circuit (IC) semiconductors which contain a number of components necessary to operate a small stepper motor. The ICs, however, are all low voltage, low current devices, which still require a separate power supply and indexer, thereby maintaining many of the problems associated with a conventional stepper motor system.
Semix, Incorporated of Fremont, Calif. offers a low voltage, low current driver for use with small stepper motors. The driver, which is in printed circuit board form, is potted in a metal enclosure that allows for better heat dissipation of the components, while decreasing EMI/RF noise radiated by the driver circuitry. The unit, however, has many limiting features. For example, the unit requires an external power supply. The step resolution that the unit can provide is limited to the number of full steps that can be achieved with a particular stepper motor. The unit requires external wiring between the driver and motor. These are only a few of the limiting aspects of Semix's driver.
It is an object of the present invention to provide a novel motor drive system that alleviates many of the problems concomitant with the motor drive systems of the prior art.
A further object of the present invention is to provide a stepper motor system that monitors the operating parameters of the stepper motor system.
Another object of the present invention is to provide a stepper motor system that is significantly reduced in size and weight from the stepper motor systems of the prior art.
A further object of the present invention is to provide a stepper motor system that has less heat generated than the heat generation of prior art stepper motor systems.
A still further object of the present invention is to provide a stepper motor system that enables significantly reduced EMI/RF emissions.
Another object of the present invention is to provide a stepper motor system in which assembly and connecting operations of the components are relatively easy to perform.
A further object of the present invention is to provide a stepper motor system that is compact in size and is effectively mounted within a small space.
Another object of the present invention is to provide a stepper motor system that is relatively simple in construction and easy to manufacture.
A still further object of the present invention is to provide a stepper motor system that has significant mechanical stability.
Another object of the present invention is to provide a stepper motor system that dissipates heat more efficiently than the prior art.
A further object of the present invention is to provide a drive package that can be easily adapted to stepper motors of various shapes and sizes.
A still further object of the present invention is to provide a stepper motor, drive circuit and control circuit in a unitary device.
Other objects and advantages of the present invention will become apparent as the description proceeds.